Several studies have shown that a large proportion of patients with major depressive disorders exhibit hyperactivity of the hypothalamic-pituitary- adrenal (HPA) axis function, expressed by elevated plasma cortisol concentrations. The goals of this study are (a) to assess HPA axis function in young children with behavior problems, and (b) to determine whether a sub-set of these children, particularly those who show concurrent symptoms of depression and anxiety may have hyperactivity of HPA axis function, as indicated by elevated plasma and salivary cortisol levels. Approximately 80 behavior problem children, aged 6-7 years, received a physical examination in order to obtain an extensive pediatric history (perinatal, developmental, and general medical history); in addition, plasma ACTH and plasma and salivary cortisol levels were assessed. Correlations between plasma and salivary cortisol levels were moderate to substantial, and cortisol levels increased under the stress of the blood drawing. Analyses of cortisol in relation to symptoms and behavior problems show inconsistent patterns. Although boys and girls do not differ on ACTH and cortisol measures, different patterns of association with symptoms and behavior problems are present. Elevated ACTH levels for boys are associated mainly with antisocial patterns, attentional, and thought problems. Elevated levels of ACTH for girls are correlated with anxiety and other internalizing problems. Some associations of ACTH and children's symptoms reported by mothers and teachers are similarly reflected in psychophysiological assessments obtained a year earlier. For boys, elevated ACTH is associated with low GSR (palmar sweat), while for girls it is associated with high GSR. Contrasting patterns of association between symptoms and these biological indices for boys and girls suggest early differences in developmental trajectories of adaptation and maladaptation.